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Davros
2005-Feb-23, 05:15 PM
I've been thinking about how we currently launch payloads into orbit and also about other methods that are being looked into such as scramjets, mid air launches etc
I had an idea of combining the good old turbojet with an oxidizer pump to create a turbojet/rocket hybrid.

Turbojets are a great and economical way of getting off the ground but obviously aren't much good once you get towards the thin upper atmosphere. So why not just bring along some extra oxidizer and start pumping it into the combustion chamber as the oxygen in the air starts to run out. This would mean you basically get an oxygen "freebie" for the first 70,000 feet or so. Needing less oxidizer at launch means you can use the weight saved for extra payload. Most turbojets use kerosene as fuel and there have been kerosene/LOX rockets used before.

The vehicle could take off like a conventional aircraft if necessary, use the jets to get up to a decent hight and velocity (say mach 3 or 4) and when it starts to run out of air use the on board oxygen to keep on going.

Now I know if it were really that simple we'd probably be doing it already, but I've looked around on the net and can't find any info on anyone having tried it before. I'm curious to see what everybodys thoughts are on the idea, what problems would need to be overcome, would it really be much of an advantage and how it would be implemented if someone tried to build one?

John Kierein
2005-Feb-23, 07:49 PM
Pegasus sorta does that.

DoktorGreg
2005-Feb-23, 07:55 PM
IIRC, Rockets are already pretty efficient for what they do. If we see improvements on them in the future, it will probably come in the form of aerospike engines, or linear aerospike engines like the one that was on the X-33.

Also, the gains that could be made to get to mach 3-4 for a hybred engine are still fairly marginal as for even just LEO you need to be going about mach 25 (sea level).

Also consider a modern rocket is already a dastardly complex piece of machinery. The liquid hydrogen tracing that lines the combustion and exhaust nozzle come to mind.

its only rocket science though:)

publiusr
2005-Feb-23, 08:19 PM
X-33 was a POS. Hydrogen demands large, simply shaped, metal tanks--not conformal, multi-lobed composite tanks. My contacts in marshall who are pro-heavy-lift knew X-33 to be a real dog. Sadly, they had little support, and X-33 had lots.

X-34 was very conventional--that is why SpaceDev wants that design for Dream Chaser.

Take a look at page 55 of this months Popular Science--with the space tourist cover.

You will see the orbital version of Dream chaser--side-mounted (so as to reduce pitch-loads and bending moments) on a large hybrid rocket with two others as strap-ons.

A miniature hybrid version of energiya buran

Good website www.up-ship.com

tofu
2005-Feb-23, 08:45 PM
I know that the problems they encountered trying to make those composite tanks work was the main thing that killed the X-33. One thing that I've always wonder about though, was why they didn't use multiple spherical tanks spread around the inside of the body.

Van Rijn
2005-Feb-23, 08:46 PM
Now I know if it were really that simple we'd probably be doing it already, but I've looked around on the net and can't find any info on anyone having tried it before. I'm curious to see what everybodys thoughts are on the idea, what problems would need to be overcome, would it really be much of an advantage and how it would be implemented if someone tried to build one?

It is a good question, but, you're right, it isn't simple and various concepts have been around for some time. Google searches on "air breathing rocket" "space scramjet" "hotol" and "skylon" will pick up quite a few references. There are several problems: Extra mass and complexity, difficulty building systems able to operate at high velocity in the atmosphere, longer time in the atmosphere and increased air resistance, higher surface temperature, and a number of others. In some cases (SpaceShipOne and Pegasus) are launched from an aircraft that acts like an air breathing first stage.

Scramjets (supersonic combustion ramjets) are being researched for the possible use in single stage to orbit rockets, but it is doubtful it will ever have an advantage over regular rockets. The extreme high temperatures require very tough materials and causes fast wear. Worse, the air resistance negates most of the advantage of gathering oxygen from the atmosphere. There are some schemes that can help (cooling the skin with fuel, similar to regenerative rocket engine cooling) but at the cost of more complexity and mass.

There have been some other proposed schemes where oxygen is gathered in various ways, but doesn't use scramjets. Here's one (HOTOL/SKYLON):

http://www.aerospaceweb.org/question/spacecraft/q0202.shtml

I wish them luck, but the essential fault with all of this is that it misses the point: Rocket fuel is the cheapest factor in a rocket launch. The real cost is in operations. If you build a reliable one or two stage truly reusable rocket, and you fly it OFTEN, it can have a pretty sad mass ratio and still be operated at costs orders of magnitude lower than the current crop. Even if you gain in mass ratio with a complex hybrid system, if the complexity makes it harder to fly, it will cost you far more to run than a good reusable system.

Davros
2005-Feb-23, 10:26 PM
Thanks Van Rijn, the link you gave me more or less answered all of my questions. I had read about HOTOL before, it seemed like a good idea but not very well implemented. Still, nice to know that people far more skilled in engineering than I have given the same kind of idea serious consideration.

I guess there is no easy solution, unless we can come up with some new materials that are very hard wearing and can withstand and/or dissapate massive temperatures any reuseable system will need a lot of refurbishment before it can be used again. Maybe nanotechnology will allow the manufacture of such materials?

Van Rijn
2005-Feb-23, 11:28 PM
I guess there is no easy solution, unless we can come up with some new materials that are very hard wearing and can withstand and/or dissapate massive temperatures any reuseable system will need a lot of refurbishment before it can be used again. Maybe nanotechnology will allow the manufacture of such materials?

"No easy solution"? Well, it depends on your goal. If your goal is much less expensive travel to orbit, the dirty little secret is that it isnt a technology problem, it is a market problem. It is hard to make flight inexpensive if you throw the rocket away after one flight. But even a great reusable design will be expensive to operate if you only fly it once or twice a year. If you create a good design AND fly it often, the cost per flight goes way, way down.

Hybrid airbreathers may have a place eventually, as may tether/momentum bank schemes. But that isn't really an issue until you have a well developed space infrastructure with hundreds of orbital flights per day.

Davros
2005-Feb-24, 03:01 AM
But if the turnaround of the reuseable rocket involves replacing significant amounts of components due to wear, that means you can't use it that often. Or are you suggesting that a scheme of massive paralellism will result in a reduced cost per unit flown?

scourge
2005-Feb-24, 09:18 AM
Conventional rockets are about 2% efficient, iirc.

Sock Munkey
2005-Feb-24, 11:16 AM
Meh, we should have been using Bob Truax's "Sea Dragon" design all along.

DoktorGreg
2005-Feb-24, 01:45 PM
Conventional rockets are about 2% efficient, iirc.

The shuttle main engines are on the order of 90%+ efficient, they are more efficient at high altitudes than at sea level . They had to design them to compensate for water vapor formation inside the nozzles. That is to say, so much heat is converted to thrust in the shuttle engines that the exhaust (water) is below 212, thus the nice smooth vapor trail you see when it launches.

Van Rijn
2005-Feb-24, 09:43 PM
But if the turnaround of the reuseable rocket involves replacing significant amounts of components due to wear, that means you can't use it that often. Or are you suggesting that a scheme of massive paralellism will result in a reduced cost per unit flown?

Properly designed, there is no reason for extremely high component wear. The issue is designing for reliability rather than maximum performance. For example, compare the Pratt & Whitney RL-10 (and other engines in the same family) to the shuttle main engines. The SME was designed for maximum performance using the technology available at the time, and pays for it in service life. This is exactly why you DON'T want to use scramjets or other hybrid designs that require absolute maximum performance.

For a spacecraft that is to lower the cost of getting into space, the design goals should be reliability, survivability, and low maintenance. New technology and maximum performance are for experimental designs, not operational ones.

tracer
2005-Feb-26, 01:11 AM
Conventional rockets are about 2% efficient, iirc.

The shuttle main engines are on the order of 90%+ efficient, they are more efficient at high altitudes than at sea level .
Caveat:

When talking about the "efficiency" of rocket engines, percentages are not a very useful number. A much more useful figure is what's called the Specific Impulse, or Isp, which is how long you can produce one pound of thrust by burning one pound of onboard fuel-and-oxidizer.

F'rinstance, a typical solid rocket booster has an Isp between 350 and 380 seconds. A hydrogen-oxygen liquid-fuelled rocket engine has an Isp of around 420 seconds. By comparison, a jet engine has an Isp on the order of 4000+ seconds.

publiusr
2005-Mar-02, 06:36 PM
I know that the problems they encountered trying to make those composite tanks work was the main thing that killed the X-33. One thing that I've always wonder about though, was why they didn't use multiple spherical tanks spread around the inside of the body.

That would have added more weight. At very large scales you can get away with big heavy spherical tanks. The huge liquid natural gas tanks were even considered for the Euclid system. The DC-X tankage was from Chicago Bridge & Iron from Birmingham, Alabama.

Sphere-packing a lifting body is asking for trouble.
LH2 likes nice, big, simply shaped metal tanks. Delta IV is just not big enough. ET's are, and at that scale they can be thick enough to act as a strongback.

The idea that "you mustn't mix crew and cargo" is a myth. You don't mix tankage with anything.

I come at this from a different place than many.

The shuttle-bashers have done their jobs too well. If you mix highly dense crew and cargo--but keep the large bulk of engines, propellant and tankage outside the airframe--you get a smarter, newer, Buran type craft big enough to have, say, an F-111 style crew escape system and extra provisions and on-orbit fuel, than a mini-spaceplane perched on top of an EELV (pitch-loads, bending moments are against you, weight-creep and cramping).

On the other hand, if you try to make an SSTO, you wind up trying to build a hypersonic dirigible--a cryogenic blimp of an eggshell.

With a mid sized craft, you have something scaled up big enough to be useful (unlike OSP/CEV) but compact enough to be buildable (like Buran but unlike X-33).

Remember, interior volume goes up with the cube as surface area goes up only with the square. Pack something with spheres, and you add more surface area. Make something conformal too large, and it gets limp. Remember, the ET doesn't need heat-shield weight. If it did, you would have to try to save weight on the tankage--spend on composites--and get burned like on the X-33 crapster.

But, when you go super big, the volume is such that you can get away with high volume tanks of massive, rugged, simple, build, like the ET, and like Sea Dragon--which must be even stronger as it is a pressure-fed rocket--not pump fed like SSME's. A bit larger, and the Euclid system with a couple of big, tough spheres is do-able. Then the volume will work for you.

There is no Moore's law to booster design. Build big--or don't build at all.


This is why I go against the flow in my advocacy of an Energiya-Buran launch system for the US. Keep the SSMEs/RS-68s on the ET, a simple orbiter with only OMS pods (where OSP would throw away many tran-stages, like Dyna-Soar), and the orbiter could be swapped out with 100 ton payload pods much simpler than Shuttle-C. The ET can remain in orbit, and be used as extra space for ET stations. We could carry our ET's to orbit--and ET based stations make a lot of sense.

www.k26.com/buran
www.spaceislandgroup.com

JHotz
2005-Mar-08, 07:40 AM
I've been thinking about how we currently launch payloads into orbit and also about other methods that are being looked into such as scramjets, mid air launches etc
I had an idea of combining the good old turbojet with an oxidizer pump to create a turbojet/rocket hybrid.

Turbojets are a great and economical way of getting off the ground but obviously aren't much good once you get towards the thin upper atmosphere. So why not just bring along some extra oxidizer and start pumping it into the combustion chamber as the oxygen in the air starts to run out. This would mean you basically get an oxygen "freebie" for the first 70,000 feet or so. Needing less oxidizer at launch means you can use the weight saved for extra payload. Most turbojets use kerosene as fuel and there have been kerosene/LOX rockets used before.

The vehicle could take off like a conventional aircraft if necessary, use the jets to get up to a decent hight and velocity (say mach 3 or 4) and when it starts to run out of air use the on board oxygen to keep on going.

Now I know if it were really that simple we'd probably be doing it already, but I've looked around on the net and can't find any info on anyone having tried it before. I'm curious to see what everybodys thoughts are on the idea, what problems would need to be overcome, would it really be much of an advantage and how it would be implemented if someone tried to build one?

Here a few things to consider.

Vertical flight is not very efficient. Therefore space vehicles fly very fast to minimize the amount of energy they expend to fight gravity. Flying very fast through the atmosphere is not very efficient either as you create an enormous supersonic wake and stress you vehicle by compounding acceleration forces in the same direction a gravity.

Horizontal flight would be more efficient except for the extra weight of wings, landing gear, a cockpit that you can see out of, and a frame strengthened to take the stresses of takeoff. I believe that fully fueled and loaded shuttle would be to heavy for any existing runway.

Rockets are the most efficient for the mad dash of vertical flight for several reasons.

High volume air intakes for thin air would be a huge drag in the denser air. Perhaps they could reconfigure as air density changes though. A rocket allows for the most efficient aerodynamics.

Rockets use pure oxidizer. Air is only one-fifth oxygen. This means that you must heat up four parts of inert gas for every part of oxidizer. The energy used to do this is no longer available to provide thrust.

Rockets engines are very compact and provide a superior thrust to weight to air breathing engines.

A major problem with jet is debris being sucked into the intakes. This happens at low speeds. When you see the launch cameras they is often a lot of junk flying around like ice and insulation chunks.


Surely you have heard of Pegasus system in which a launch vehicle is carried up by a B-52. The problem is this system will not work for heavy payloads. My favorite is the use of a heavy lift balloon.

Kaptain K
2005-Mar-08, 11:40 AM
Rockets use pure oxidizer. Air is only one-fifth oxygen. This means that you must heat up four parts of inert gas for every part of oxidizer. The energy used to do this is no longer available to provide thrust.

This is incorrect! Thrust is produced by the expansion of hot gases. The nitrogen expands when heated just like the combustion products do.

publiusr
2005-Mar-08, 10:45 PM
We just need to get on the HLLV bandwagon--and Shuttle-derived seems to be the #1 contender to fight EELV with.

JHotz
2005-Mar-12, 04:21 AM
Rockets use pure oxidizer. Air is only one-fifth oxygen. This means that you must heat up four parts of inert gas for every part of oxidizer. The energy used to do this is no longer available to provide thrust.

This is incorrect! Thrust is produced by the expansion of hot gases. The nitrogen expands when heated just like the combustion products do.Thank you for responding to this post.

You are correct thrust is created by expansion. The efficiency of a rocket is directly proportional to the average speed of the exhaust particles. Heat is a measure of the average speed of the particle in the medium. If you put a bunch of cold inert gas into the reaction it will lower the overall temperature of the reaction and therefore the average exhaust speed.

A Thousand Pardons
2005-Mar-13, 11:13 PM
Rockets use pure oxidizer. Air is only one-fifth oxygen. This means that you must heat up four parts of inert gas for every part of oxidizer. The energy used to do this is no longer available to provide thrust.

This is incorrect! Thrust is produced by the expansion of hot gases. The nitrogen expands when heated just like the combustion products do.Thank you for responding to this post.

You are correct thrust is created by expansion. The efficiency of a rocket is directly proportional to the average speed of the exhaust particles. Heat is a measure of the average speed of the particle in the medium. If you put a bunch of cold inert gas into the reaction it will lower the overall temperature of the reaction and therefore the average exhaust speed.
yabut I think Kaptain K was pointing out that the energy is still available for thrust--but you start with less fuel.

JHotz
2005-Mar-14, 12:30 AM
yabut I think Kaptain K was pointing out that the energy is still available for thrust--but you start with less fuel.
Thank you for responding to my post.

The energy is not still available because it was used to heat up the inert gas. Gas welders use pure oxygen instead of a compressor because they get a much hotter flame by removing the inert gas from the reaction? Heat is energy. Inert gas will take energy thus cooling the reaction thus reducing the exhaust speed. Yes the inert gas is thrust out of the rocket. Yes it contributes to thrust. The point is you get significantly less thrust overall because the inert gas did not contribute to the energy of the reaction like more fuel and oxygen would have.

A Thousand Pardons
2005-Mar-14, 08:15 AM
Yes the inert gas is thrust out of the rocket. Yes it contributes to thrust.
That was Kaptain K's point, I believe, but I could be wrong. He may be on weekend.

JHotz
2005-Mar-14, 11:45 AM
Yes the inert gas is thrust out of the rocket. Yes it contributes to thrust.
That was Kaptain K's point, I believe, but I could be wrong. He may be on weekend. The good Kaptain’s point was that he believed the use of pure oxidizer offered no advantages over using normal air. I could be wrong as well but why else would he be making the point.

JHotz
2005-Mar-14, 12:35 PM
We just need to get on the HLLV bandwagon--and Shuttle-derived seems to be the #1 contender to fight EELV with.

I do not understand what you are saying. What is HLLV and EELV

Sock Munkey
2005-Mar-14, 05:09 PM
Horizontal flight would be more efficient except for the extra weight of wings, landing gear, a cockpit that you can see out of, and a frame strengthened to take the stresses of takeoff. I believe that fully fueled and loaded shuttle would be to heavy for any existing runway.

Once you hit supersonic speed wings aren't needed. The pressure of air against the hull is enough to stay airborne.

JHotz
2005-Mar-14, 09:04 PM
Once you hit supersonic speed wings aren't needed. The pressure of air against the hull is enough to stay airborne.That is an excellent point. However most planes cannot take off at those speeds. That is the paradox of wings. They must function through a whole range of loads at range of speeds and altitudes. Some wings have a variable geometry(swing wings) like the B-1 bomber. Commercial airlines have flaps to increase the lift on takeoff and landing.

One possibility is the Daisy Clipper. It can take off vertical then fly horizontal.

HypersonicMan
2005-Mar-14, 09:24 PM
We just need to get on the HLLV bandwagon--and Shuttle-derived seems to be the #1 contender to fight EELV with.

I do not understand what you are saying. What is HLLV and EELV

Heavy Lift Launch Vehicle (think Saturn V) and Evolved Expendable Launch Vehicle (Delta IV and Atlas V). publiusr has a one track mind when it comes to HLLV :D

publiusr
2005-Mar-16, 07:25 PM
I have to. Advocacy may seem a monomania--but remember why we had a space race at all:

Korolov could have waited, convinced Stalin to wait until nukes were shunk so that smaller ICBMs could have been built. Instead, korolov saw an opportunity to build a space booster bigger than what a lot of military men wanted--and sold it as an ICBM. The result was the R-7 sputnik launch vehicle that was more powerful than any all-liguid rocket we ever produced--save the Saturns and the Delta IV 'heavy.'

Without Korolov's 'one-track' mind, we wouldn't have space programs of any kind.


Heavy Lift future for Russia? (only if they and the Ukrainians work things out):

http://www.russianspaceweb.com/

A unique book on history of the world manned space exploration published recently in Russia. The book is titled "The world manned space exploration. History. Technology. People."


http://english.pravda.ru/main/18/90/363/15107_book.html